What steps will reproduce the problem?

I haven't had a chance to extract a small test case yet. In the meantime:

1. go get github.com/josharian/kart
2. cd $GOPATH/src/github.com/josharian/kart
3. git checkout code-snippet-panic 
4. present
5. open "http://localhost:3999/kart.slide#1";

What is the expected output? What do you see instead?

2014/01/21 10:15:17 http: panic serving 127.0.0.1:64105: runtime error: slice bounds out
of range
goroutine 534 [running]:
net/http.func·009()
    .../src/pkg/net/http/server.go:1093 +0xae
runtime.panic(0x319d40, 0x76650a)
    .../src/pkg/runtime/panic.c:248 +0x106
code.google.com/p/go.tools/present.codeLines(0xc210174000, 0xbb0, 0xdb0, 0x755, 0x6a2,
...)
    .../code.google.com/p/go.tools/present/code.go:186 +0x5ae
code.google.com/p/go.tools/present.parseCode(0xc2101483a0, 0xc21013f920, 0xa, 0x70,
0xc210141480, ...)
    .../code.google.com/p/go.tools/present/code.go:99 +0x943
code.google.com/p/go.tools/present.parseSections(0xc2101483a0, 0xc21013f920, 0xa,
0xc21016dfa0, 0xb77a68, ...)
    .../code.google.com/p/go.tools/present/parse.go:356 +0x18a8
code.google.com/p/go.tools/present.(*Context).Parse(0xc2101483a0, 0x8ab8f0,
0xc210148398, 0xc21013f920, 0xa, ...)
    .../code.google.com/p/go.tools/present/parse.go:263 +0x207
code.google.com/p/go.tools/present.Parse(0x8ab8f0, 0xc210148398, 0xc21013f920, 0xa, 0x0,
...)
    .../code.google.com/p/go.tools/present/parse.go:273 +0x7b
main.parse(0xc21013f920, 0xa, 0x0, 0x0, 0x0, ...)
    .../code.google.com/p/go.talks/present/dir.go:99 +0xdb
main.renderDoc(0x8acc38, 0xc2101866e0, 0xc21009f910, 0x40, 0xc21013f920, ...)
    .../code.google.com/p/go.talks/present/dir.go:66 +0x50
main.dirHandler(0x8acbd8, 0xc2101866e0, 0xc2100f3a90)
    .../code.google.com/p/go.talks/present/dir.go:33 +0x204
net/http.HandlerFunc.ServeHTTP(0x43ced8, 0x8acbd8, 0xc2101866e0, 0xc2100f3a90)
    .../src/pkg/net/http/server.go:1220 +0x40
net/http.(*ServeMux).ServeHTTP(0xc21001e600, 0x8acbd8, 0xc2101866e0, 0xc2100f3a90)
    .../src/pkg/net/http/server.go:1496 +0x163
net/http.serverHandler.ServeHTTP(0xc21009fbe0, 0x8acbd8, 0xc2101866e0, 0xc2100f3a90)
    .../src/pkg/net/http/server.go:1597 +0x16e
net/http.(*conn).serve(0xc210136f00)
    .../src/pkg/net/http/server.go:1167 +0x7b7
created by net/http.(*Server).Serve
    .../src/pkg/net/http/server.go:1644 +0x28b